A drift-tube linear accelerator is configured such that one or more pairs of hollow cylindrical drift-tube electrodes are arranged along the beam traveling direction in the cylindrical cavity. Radio-frequency electric power is fed into the cylindrical cavity and a radio-frequency electric field generated between the drift-tube electrodes accelerates charged particles (for example, such as protons or carbon ions) in the beam traveling direction. The drift-tube electrodes are designed to be arranged so that the charged particles exist inside the drift-tube electrodes when the radio-frequency electric field points toward the direction inverse to the beam traveling direction.
There are two types of electromagnetic field modes generated in the cylindrical cavity: TM mode (an electric field is generated in the longitudinal direction of the cylindrical cavity) and TE mode (a magnetic field is generated in the longitudinal direction of the cylindrical cavity). A drift-tube linear accelerator using the TM mode includes an Alvarez drift-tube linear accelerator. In this Alvarez drift-tube linear accelerator, since the electromagnetic field mode in the cylindrical cavity is used intact for an accelerating and focusing electric field to be generated between the drift-tube electrodes, the drift-tube electrodes are supported suspendedly from the cylindrical cavity. On the other hand, a drift-tube linear accelerator using the TE mode includes an interdigital H-mode (IH) drift-tube linear accelerator and the like. In the IH drift-tube linear accelerator, since the electromagnetic field mode in the cylindrical cavity cannot be used intact for an accelerating and focusing electric field to be generated between the drift-tube electrodes, the drift-tube electrodes is supported by stems that are alternately arranged vertically (or horizontally) in the cylindrical cavity, to generate indirectly an accelerating and focusing electric field between the drift-tube electrodes by an induced current.
When radio-frequency power of a predetermined frequency is introduced into the cavity, a resonance occurs and an electric field is generated between the drift-tube electrodes. By the electric field generated between the drift-tube electrodes, particles are accelerated increasingly every time passing through the drift-tube electrode.
Since a particle beam is aggregation of charged particles, a repulsive force acts between each other of the particles (this refers to as “space charge effect”). For that reason, the particles spread in both radial and traveling directions as they travel in the traveling direction; in particular, the radial spread causes loss of the particles due to collision with the vacuum duct wall. Hence, there is needed a beam radially focusing device that suppresses radial spreading of the beam. Conventionally, the beam spreading has been suppressed using a focusing device built-in drift-tube electrode that incorporates a focusing device and a drift-tube electrode (Patent Document 1). However, the alternating phase focusing (APF) method is recently proposed, in which a beam focusing force is obtained by a design that couples generation of curved electric-field distribution between drift-tube electrodes and the timing for charged particles to travel therebetween (Patent Document 2).
An APF-IH linear accelerator, which is fabricated by applying the APF method to an IH linear accelerator, eliminates the need for using such a focusing device built-in drift-tube electrode, achieving low cost and simple structure; and hence has been used, for example, in a field such as of medical devices necessary for reliability.
In a medical synchrotron facility using a heavy particle beam such as of carbon ions (not include protons), an APF-IH linear accelerator is utilized as a subsequent stage accelerator to the injector. Carbon ions produced by an ion source are pre-accelerated through a former stage accelerator, and then focused by three successive quadrupole electromagnets so as to satisfy an injection condition (acceptance) of the APF-IH linear accelerator. After that, the injected tetravalent carbon beam of 400 eμA (=100 μA) is accelerated up to 4 MeV/u. By employing the APF-IH linear accelerator, compactness of about ⅙ in total length is achieved compared to a conventional drift-tube linear accelerator (Alvarez drift-tube linear accelerator) that uses a focusing device built-in drift-tube electrode (Non-Patent Document 1).